Possibility of Seedless ‘Koyama New Summer’ Fruit Production by Fruit Thinning under Natural Pollination Conditions

Abstract

Koyama New Summer' is a bud mutation of 'Hyuganatsu' (Citrus tamurana hort. ex. Tanaka). We examined the possibility of using fruit thinning to increase the production of seedless 'Koyama New Summer' fruit under natural pollination conditions. We found that 40% of the fruit produced in the field were seedless under natural pollination conditions. Examination of the relationship between the seed number and fruit quality showed that fruit with more seeds were larger in size, had a lower Brix value, and had thicker peel. Furthermore, many of the fruit with a prominent nipple at the apex contained seeds. Consequently, we considered that the transverse diameter of the fruit and presence of a prominent nipple at the apex could be used as indices for fruit thinning to increase seedless fruit production. We estimated the number of seeds and seedless fruit ratio in the harvested fruit by these two indices of fruit thinning. There was a high seedless fruit ratio in the harvested fruit when fruit thinning was undertaken in July by the prominent nipple at the apex of the fruit compared with no fruit thinning. It was considered that the prominent nipple at the apex of the fruit is easier to use as an index of fruit thinning than the transverse diameter of the fruit when having to decide on the criterion for annual fruit thinning. In conclusion, we recommend that fruit thinning is performed in July using the prominent nipple at the apex as an index to remove a part of seeded fruit, and so increase seedless fruit production.